Identification transponders are often used to monitor or identify animals such as cattle and other ruminants. Identification transponders are generally either attached to the ear or reside inside the animal's digestive system (ie. in the reticulum or rumen). When an identification transponder is within the influence of a sufficiently transmitted field strength, having a specific frequency range, it becomes energised and transmits a coded signal. The signal is either received by the antenna providing the field or by another suitably positioned receiving antenna. This signal identifies the animal which may then be weighed for monitoring purposes.
One type of antenna arrangement uses a hand held antenna which is moved towards an animal to eners and receive a coded signal from an identification transponder associated with the animal. This is a labour intensive task as it requires a person to manoeuvre the antenna into suitable positions. Further, this can be impractical or dangerous when used in passageways such as cattle runs in which animals may jump or bolt.
Stationary antenna arrangements have also been developed and include antennas positioned on one or both sides of a passageway and may also include an antenna positioned across the passageway.
Disclosed in U.S. Pat. No. 3,493,955 is a stationary antenna arrangement comprising a transmitter antenna and a receiver antenna positioned on opposite sides of a passageway. A further antenna is positioned under the floor and is positioned across the passageway.
U.S. Pat. No. 3,838,409 discloses a stationary antenna arrangement including three stationary antennas, two being positioned on opposite sides of a passageway and the third forming a frame around a section of the passageway. The antennas positioned on opposite sides of the passageway are electrically connected to both a first output of a transmitter and an input of a first detector. The third antenna is electrically connected to a second output of the transmitter which provides a signal out of phase with the first output. The third antenna is also connected to an input of a second detector. This arrangement minimises the effects of regions of minimal electromagnetic field intensity.
In U.S. Pat. No. 4,963,880 there is disclosed an antenna arrangement including a stationary single coil co-planar antenna typically 10 inches in diameter. The antenna is electrically connected to transmitting and receiving circuitry in which the antenna alternates from transmit mode to receive mode. U.S. Pat. No. 5,274,392 discloses a stationary antenna arrangement including a stationary antenna disposed around a conveyor belt in a plane at an acute angle to both the horizontal and the longitudinal axis of the conveyor belt. In U.S. Pat. No. 4,527,152 there is disclosed a system comprising two opposing rectangular antennas inclined at 25.degree. to the horizontal. However, the antennas can provide an interrogation field(s) with a "nulled" or reduced field strength region due to opposing fields from each antenna being transmitted into the interrogation region. There are many other patents disclosing stationary antenna arrangements which include U.S. Pat. No. 3,500,373, U.S. Pat. No. 4,845,509, U.S. Pat. No. 4,633,250 and U.S. Pat. No. 5,142,292.
One problem with the stationary antenna arrangements disclosed in the above U.S. patents is that they are neither specifically designed nor ideally suited for applications such as animal identification or other applications in which the position of the transponder may vary significantly in orientation, height, distance from a passageway side or speed through the passageway. All of these variables may exist in animal identification due to the size of the animal, location of the transponder and speed of the animal (which may bolt past the antenna arrangement). A further problem is due to Government regulations restricting the amount of power that may be transmitted.
The above problems in combination with the shape of the field patterns provided by the antenna or antennas has the disadvantage of the transmitted field not always being sufficient to energise an animal identification transponder and receive a coded signal therefrom. Accordingly, animals may have to be held stationary, reversed or manoeuvred in relation to the antenna arrangement and when a single antenna is used then the animal may have to be pushed towards the antenna. The above solutions are both labour intensive and physically demanding.